Thiophene/selenophene-based S-shaped double helicenes: regioselective synthesis and structures

2,5-Di(trimethylsilanyl)dithieno[2,3-b:3′,2′-d]thiophene ((TMS)2-bb-DTT), 2,5-di(trimethylsilanyl)diseleno[2,3-b:3′,2′-d]thiophene ((TMS)2-bb-DST), and 2,5-di(trimethylsilanyl)diseleno[2,3-b:3′,2′-d] selenophene ((TMS)2-bb-DSS) were used as starting materials to synthesize three S-shaped double helicenes (i.e., DH-1, DH-2, and DH-3) through monobromination, formylation, the Wittig reaction, and double oxidative photocyclization. The photocyclization was a highly regioselective process. The molecular structures of DH-1 and DH-2 were confirmed by X-ray single-crystal analysis. Multiple intermolecular interactions, such as C–S, C–Se, S–S, S–Se, and Se–Se, were observed in the crystal packing structures of these compounds. Spectroscopic results and our previous work showed that the combination of molecular structure change and heteroatom replacement from S to Se could precisely modulate molecular energy levels.

As its close analogue, selenophene has properties very similar to those of thiophene. Fused aromatic compounds containing selenophene units show favorable optical and electrochemical properties and improved charge transport characteristics in the solid state mainly because such fused aromatic compounds often undergo increased Se-Se interactions, which confer ordering at the molecular scale and, thus, lead to well-aligned solid-state packing and excellent charge-transport properties [20,21].
In this synthetic study, regioselective double oxidative photocyclization was observed during the construction of three S-shaped double helicenes DH-1-3 based on thiophene/ selenophene. From DH-1 to DH-3, sulfur atoms in the molecular framework were gradually replaced by selenium atoms. The crystal structures of DH-1 and DH-2 and spectroscopic features of DH-1-3 were then studied. Finally, the reaction sites of oxidative photocyclization, energy levels, and the electron cloud distribution of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs) are predicted.

5-(Trimethylsilyl
Compounds 5 had five isomers and three reaction sites (2, 4, and 6-positions in the benzene moiety) during oxidative photocyclization. Irradiation of 5a-c resulted in oxidative photocyclization products with two types of configurations wherein two benzene rings were closed in the same and opposite direction, such as DH-1-3 and 6 ( Figure 2). However, after the double oxidative photocyclizations of 5a-c in the presence of iodine and propylene oxide in dry toluene through irradiation by a 450 W Hg medium-pressure lamp for 1.5 h, only one type of ring-closing product with two benzene rings formed in the same direction, i.e., S-shaped double helicenes DH-1-3 were obtained in yields of 62%, 30%, and 53%, respectively ( Figure 2).
The double oxidative photocyclization reaction sites of 5 were predicted by the orbital-weighted Fukui function in Multiwfn using Gaussian 09 [31,32] at the B3LYP/6-31G** [33] level of theory to verify the reaction-site selectivity of oxidative photocyclization further. Results showed that the conformations of 5a are varied, but the orbital-weighted Fukui function is not connected to conformation. In the orbital-weighted Fukui function, the larger the isosurface distribution, the higher the activity of reactions. Thus the 4 and 6-positions of benzene are the most likely sites for reaction (see Supporting Information File 1, Figure S21). However, after the formation of the naphthalene ring, the α-position is the most likely site for reaction (Supporting Information File 1, Figure S22). Therefore, after the double oxidative photocyclization of compound 5a, product DH-1 is mainly obtained. The predicted result of the double oxidative photocyclizations of 5a is consistent with the experimental result, that is, an S-shaped double helicene can be selectively obtained through the double oxidative photocyclization of compound 5a.

Crystallographic analyses of DH-1 and DH-2
The molecular structures of DH-1 and DH-2 were confirmed by single-crystal analysis ( Figure 3). Both DH-1 and DH-2 belong to the triclinic space group P-1. After double oxidative photocyclizations of 5a and 5b, DH-1 and DH-2 are compressed into S-shaped double helical structures ( Figure 3A and B), which consist of one [5]helicene and one [6]helicene. The two helicenes have the same configuration and bend toward the same direction on the same side of the shared naphthalene ring ( Figure 3C and D). Both products DH-1 or DH-2 feature a pair of enantiomers MM and PP in their unit cell (see Supporting Information File 2, Figures S2 and S6). The crystal parameters of DH-1 and DH-2 are shown in Table 1. The replacement of sulfur with selenium in DH-1 and DH-2 leads to turn angles in-plane and helix climbs of [5]helicene and [6]helicene of DH-1 and DH-2 significantly change (Table 1, Figure 3).
Each of the two DH-1 molecules (blue and red molecules in Figure S3 of Supporting Information File 2) in the unit cell interacts with six adjacent molecules. For example, in Figure Figures S4 and S8 in Supporting Information File 2), which is a suitable characteristic for helicene compounds used as supramolecular self-assembly units [34][35][36][37][38][39][40][41].

Spectroscopic features of DH-1-3
The UV-vis absorption spectra of DH-1-3 in dichloromethane are shown in Figure 4. In general, the UV-vis absorption spectra of DH-1-3 are generally similar in shape and exhibit three major absorption bands within 230-280 nm (band-I), 280-330 nm (band-II), and 304-414 nm (band-III) (Figure 4). Progressive red-shifts in the absorption spectra of band-I, band-II, and band-III occur with increasing number of selenium atoms. In band-I, compounds DH-1-3 show a maximum absorption peak at 232, 240, and 242 nm, respectively. In band-II and band-III, helical distortion and possible conjugation through heteroatoms (e.g., sulfur and selenium atoms) in DH-1-3 may increase π-electron delocalization, leading to a  red-shifted broad absorption. The maximum absorption peaks of DH-1-3 appear at 268, 275, and 279 nm, respectively, in band-II and at 323, 331, and 336 nm, respectively, in band-III. Thus, the optical band gaps estimated from the absorption edges gradually decrease from DH-1 to DH-2 to DH-3, and are equal to 3.08, 3.01, and 2.98 eV, respectively. This change trend is consistent with the calculated results, which are 3.97, 3.83, and 3.81 eV for DH-1, DH-2, and DH-3, respectively (see Table S2 in Supporting Information File 1). However, the optical band gaps of 1, 2, and DH-3 obviously differ because of changes in their molecular configuration and equal to 2.86, 3.15, and 3.81 eV, respectively [27,28]. As the number of selenium atoms increases from DH-1 to DH-3, the fluorescence intensity ( Figure S19 in Supporting Information File 1) and the fluores-cence quantum yield (Φ F , Figure S20, Table S1 in Supporting Information File 1) of the molecules also decrease ( Figure 4).

Conclusion
In summary, the key step of regioselective double oxidative photocyclization was successfully employed in the preparation of three S-shaped double helicenes, namely, DH-1, DH-2 and DH-3 with (TMS) 2 -bb-DTT, (TMS) 2 -bb-DST, and (TMS) 2 -bb-DSS as starting materials. The synthetic method described in this research not only provides a method for the synthesis of Sshaped double helicenes but also enriches the family of selenophene helicenes. Multiple intermolecular interactions and regular arrangement in the crystal packing structures of DH-1 and DH-2 indicate that these compounds may be used as supra- molecular self-assembly units. Changes in molecular structure may substantially modulate the optical band gap of heteroacenes, and the replacement of heteroatoms from S to Se could fine-tune their optical band gap [18,27,28]. Thus, the combination of molecular structure modification and atom replacement could be a viable strategy, for the precise modulation molecular energy levels and yield molecules with strong application potential in organic functional materials, such as OFETs, and CPLs, among others.

Experimental General procedures and materials
Tetrahydrofuran (THF) for use on vacuum line was freshly distilled from sodium/benzophenone prior to use. n-BuLi (hexane) were obtained from Energy Chemical; prior to use, its concentration was determined by titration with N-pivaloyl-otoluidine [42]. Column chromatography was carried out on silica gel (300-400 mesh). Analytical thin-layer chromatography was performed on glass plates of silica gel GF-254 with detection by UV. HRMS analysis was carried out on a mass spectrometer equipped with DART-FT-ICR and MALDI-TOF-CHCA. Melting-point determination was taken on a Melt-Temp apparatus and mp are uncorrected. The X-ray crystallographic analyses were performed using crystals of compounds DH-1 and DH-2 with sizes of 0.14 × 0.12 × 0.08, 0.21 × 0.17 × 0.12 mm 3 , respectively. The intensity data were collected with the ω scan mode (296 K) on a diffractometer with a CCD detector using Cu Kα radiation (λ = 1.54184 Å). The data were corrected for Lorentz and polarization effects, and absorption corrections were performed using the SADABS program [43]. The crystal structures were solved using the SHELXTL program and refined using full-matrix least-squares [44]. .05 equiv) in THF (5 mL) was added at −78 °C, the mixture kept for 1 h, and then the reaction mixture was warmed up slowly to ambient temperature overnight. The reaction mixture was quenched with CH 3 OH and extracted with CH 2 Cl 2 (3 × 10 mL). The organic layer was washed with saturated NaCl (20 mL) and water (2 × 20 mL), and then dried over MgSO 4 . The residue was purified by column chromatography (eluent: hexane/CH 2 Cl 2 3:1 (v/v) and recrystallized from CHCl 3 / CH 3 OH to yield 5a (20.9 mg, 46%) as a yellow solid; mp > 300 °C; 1

Synthesis of DH-1
To a solution of 5a (9.6 mg, 0.014 mmol) in dry toluene (6 mL), iodine (7.3 mg, 0.028 mmol, 2.0 equiv) and excess propylene oxide were added. The reaction solution was irradiated with a 450 W unfiltered Hg medium-pressure lamp for 1.5 h. The reaction was quenched with saturated Na 2 SO 3 solution (5 mL